1. Field of the Invention
This invention relates to a rotary drill bit for use in the formation of wellbores. In particular, the invention relates to a rotary drag-type drill bit of improved stability, particularly when worn. A drill bit of this type typically comprises a bit body having a shank for connection to a drill string, a plurality of blades formed on the bit body extending outwardly from a central axis of rotation of the bit, and a plurality of cutting elements mounted along each blade.
2. Description of the Prior Art
The bit body may be machined from solid metal, or may alternatively be molded using a powder metallurgy process in which a tungsten carbide powder is infiltrated with a metal alloy binder in a furnace so as to form a hard matrix.
Typically, the cutters each take the form of a tablet of superhard material bonded to a substrate, for example of tungsten carbide. Each cutter is typically of circular or part-circular shape.
In some drill bits, the cutters are arranged upon the blades at different radial distances to one another so that the cutters sweep over the full area of the bottom of the wellbore. However, there is a tendency for drill bits of this type to be of relatively low stability.
In order to improve the stability, it is known to arrange the cutters in a series of concentric rings at different cutting heights. As a result, the drill bit tends to form, in the bottom of the wellbore, a series of corrugations or rings. The co-operation between the drill bit and the pattern formed in the bottom of the wellbore tends to resist lateral movement of the bit, resulting in the drill bit being of improved stability. In the past, drill bit designs were compromised because cutters having high abrasion resistance had low impact toughness and cutters having high impact toughness had low abrasion resistance. As a result of this trade-off such placement of the cutters may improve the abrasion resistance of the drill bit, but it is likely that the overall drilling efficiency is not optimized due to relatively poor impact toughness.
Drill bits of this type are known as tracking drill bits and are described in, for example, GB 2294712 and GB 2292163.
It is an object of the invention to provide a drill bit of good stability, particularly when the drill bit becomes worn.
According to the present invention there is provided a rotary drill bit comprising a bit body having an axis of rotation, a plurality of cutters mounted upon the bit body in a plurality of concentric rings centered upon the axis of rotation, wherein said plurality of cutters comprises a first type of cutter of relatively low abrasion resistance and a second type of cutter of relatively high abrasion resistance, at least one of said concentric rings consisting of cutters of the first type, at least another of said concentric rings including at least one cutter of said second type. The impact toughness of all the cutters is substantially the same.
It will be appreciated, in use, that the cutters of the first type will wear at a higher rate than those of the second type with the result that a drill bit which initially has a reasonably uniform cutting profile will, when worn, form grooves in the formation being drilled. As a result, the stability of the drill bit improves with wear.
Preferably, the cutters of the second type each include a polycrystalline diamond table treated so as to render a region thereof close to a cutting edge thereof substantially free of a material having a catalyzing effect. Cutters of this type have been found to have an improved wear and abrasion resistance when compared with cutters not so treated and yet have substantially the same impact toughness as the first type of cutters.
According to another aspect of the invention there is provided a rotary drill bit comprising a bit body having a leading face and an axis of rotation, the leading face having a plurality of angularly spaced blades, each said blade carrying a plurality of cutters, wherein said plurality of cutters includes a first type of cutter having a relatively low abrasion resistance and a second type of cutter having a relatively high abrasion resistance, and wherein at least one of said blades carries at least one cutter of the first type and at least one cutter of the second type. The impact toughness of all the cutters is substantially the same.
Preferably, the cutters on each blade of the first type are arranged alternately with cutters from the second type.